Recognition system

ABSTRACT

A recognition system comprising a transmitter for radiating an amplitude modulated signal to a target that generates a harmonic of the received signal, detects the amplitude modulating signal and modulates the harmonic signal with the detected signal. The target contains a frequency selective network that produces a coded arrangement of signal components which comprise the signal returned by the target. A receiver detects the coded arrangement of signal components.

United States Patent 1191 Augenblick et al.

1451 Mar. 19, 1974 RECOGNITION SYSTEM Inventors: Harry A. Augenblick,Mountain Lakes; William J. Engle, Ridgewood, both of NJ.

Related U.S. Application Data Continuation-impart of Ser. No. 85,127,Oct. 29, 1970, abandoned.

U.S. Cl. 343/7 ED, 340/258 R, 343/65 SS,

3,299,424 l/1967 Vinding 340/258 C 3,447,154 5/1969 Schrader 343/65 R3.l28,4l6 4/1964 Rode 340/258 C 3,384.892 5/1968 Postman 343/65 R3,182,314 5/1965 Kleist et a1.. 343/65 SS 3,060,425 10/1962 Cutler 343/7ED Primary ExaminerJohn W. Caldwell Assistant Examiner--Marshall M.Curtis Attorney, Agent, or FirmRyder, McAulay, Fields, Fisher &Goldstein 5 7] ABSTRACT A recognition system comprising a transmitterfor ra- 343/225 diating an amplitude modulated signal to a target that51 Int. Cl. G0ls 9/56 generates a harmonic of the received Signal,detects 58 Field of Search 340/280, 258 R, 258 c; the amplitudemodulating Signal and modulates the 343/225, 65 R, 6.5 55, 7 325/8harmonic signal with the detected signal. The target contains afrequency selective network that produces [56] References Cited a codedarrangement of signal components which UNITED STATES PATENTS comprisethe signal returned by the target. A receiver detects the codedarrangement of signal components. 3,631,484 12/1971 Augenblick 343/65 R3,108,275 10/1963 Chisholm 343/65 R 28 Claims, 18 Drawing FiguresOSCILLATOR 22 wo 1 I4 g 1 2 f (/6 I8 20 1 RF DlRECTIONAL AMPLITUDE 24 IOSCILLATOR COUPLER MODULATOR 1 26 L 32 "'1 FREQUENCY e44 DOUBLER 1 42 40as I 35 l 1 m 48 m 1 i I TUNED TUNED AMPLIFIER AMPLIFIER 11 I 1 1 5o 521 DETECTOR I 1 I AMPLITUDE -54 1 COMPARATOR 1 l i 1 56 i .1

PAIENIEDIIIR I 9 I974 3. 798,642

SHEET 1 OF 4 OSCILLATOR l4 I I r RF DIRECTIONAL AMPLITUDE OSCILLATORCOUPLER MODULATOR FREQUENCY 44 DOUBLER 38/ MIXER FILTER I l l TUNEDTUNED I AMPLIFIER AMPLIFIER I1 I I I I I I I I I l I I I I 50w I r I I II I I I I I I DETECTOR DETECTOR AMPLITUDE 54 COMPARATOR F IG. 2A. FIG.25. F IG. 2C.

I I I UJ 1 I.|.I g S 8 I- t I: .J -I I I I g I I 5:. I I I I I I I -f FF+ ZF-ZfZF-F 2F 2F+f 2F+2 2F-2; 2F-,c 2F 2F+5=2F+25 FREQUENCY FREQUENCYFREQUENCY 82 FIG. 98. I h 84 as I INVENTORS I HARRY A. AUGE NBLICKMODULATION FREQUENCY f ENGI-E AMPLITUDE ATTORNEY.

1 RECOGNITION SYSTEM This application is a continuation-in-part of ourUS. application Ser. No. 85,127, filed Oct. 29, 1970, now abandonedentitled Detection System.

This invention relates generally to a recognition system for identifyingone or more groups of harmonic generating targets and more particularly,pertains to personnel and object identification systems and the like.

Various techniques have been developed in the past for monitoringentrances to prevent unauthorized entry of persons or the unauthorizedremoval of merchandise. Some of these techniques utilize passiveharmonic generating targets such as diodes that are embedded in orotherwise attached to an entry badge or a protected article. As thearticle or person wearing the badge enters a surveillance area, atransmitter sends a signal of fundamental frequency to the harmonicgenerator. The harmonic generating targets operate to radiate harmonicsof the fundamental signal. The target and thus the article to which itis attached is thereupon detected by the reception of harmonic energyemanating from the area under surveillance. More specifically, harmonicreceivers are usually provided to monitor such areas to and sound analarm when such harmonic energy is detected.

One major difficulty encountered in connection with prior systems of theabove-described types lie in the fact that numerous objects that arefound in typical environments of the type under consideration arecapable of generating harmonics of the fundamental signal wheninterrogated. For example, all electronic equipment containing diodes,transistors, or other semiconductors, such as transistor radios, taperecorders, phonographs, hearing aids, certain cameras and otherphotographic accessories, certain watches, and the like will reradiatesuch harmonic energy and thereby actuate an alarm, even though thepassage of such objects is proper and authorized.

In addition to such manufactured diodes, transistors, and othersemi-conductors, any metal-to-metal junction is capable of generatingharmonics under certain conditions. Chain link fences, wire screening,loose coins, keys, paper clips, and countless other metal objects willconvert a portion ofa pure fundamental signal into harmonic energy,reradiate this harmonic energy, and cause an alarm signal to be emitted.

Accordingly, an object of the present invention is to provide animproved recognition system.

A more specific object of this invention is to provide a recognitionsystem which will recognize and track a particular target to theexclusion of false tracking of similar targets.

Another object of the present invention is to provide a recognitionsystem to identify a particular harmonic generating target or class oftargets.

A further object of the present invention is to separately recognize twoor more classes of harmonic generating targets.

A recognition system constructed according to the present inventioncomprises a transmitting unit which transmits an amplitude modulatedsignal. Target means receives the transmitted signal and generates aharmonic signal comprising a plurality of signal componets havingrespective amplitudes. The target means further comprises amplitudevarying means for varying the amplitude of at least one of the pluralityof signal components to provide an output signal having a preselectedcoded arrangement of the amplitudes of the plurality of signalcomponents. Receiving means receives the output signal and comprisessensing means which produces a recognition signal when the preselectedcoded arrangement of amplitudes of the plurality of signal components isreceived.

Other features and advantages of the present invention will become moreapparent from a consideration of the following detailed description,when taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of a system constructed according to thepresent invention:

FIGS. 2A, 2B and 2C illustrate, in graphical form, the frequencyspectrum associated with the system shown in FIG. 1;

FIG. 3 illustrates a circuit wiring diagram, in block form, of amodified embodiment of a system constructed according to the invention;

FIGS. 4A, 4B and 4C illustrate, in graphical form, the frequenciesassociated with the system shown in FIG.

FIG. 5 illustrates a circuit diagram, in clock form, of a furthermodified embodiment of a system;

FIG. 6 illustrates, in graphical form, the frequencies associated withthe system shown in FIG. 5;

FIGS. 7A, 78, 8A, 8B 9, l0 and 11 illustrate, in schematic and graphicalform, respective embodiments of harmonic generating targets withdifferent frequency selective networks, forming a portion of the systemof the invention; and

FIG. 12 is a block diagram of a modified embodiment of a transmitterforming a portion of the system of the invention.

As noted hereinabove, the present invention includes a harmonicgenerating target. While a number of elements can generate harmonics,the present invention will be described in conjunction with asemiconductor diode. However, this is not to be interpreted as being alimitation on the present invention since any device which generatesharmonic signals from a received signal of fundamental frequency may beutilized. Moreover, as noted below, the present system includesdetection of the amplitude modulating signal transmitted to the target.Hence, the diode should also provide efficient detection at thefrequencies of interest.

Certain specific frequencies of operation will be specified hereinbelow.However, it is again emphasized that these ranges of frequencies are forillustrative purposes only and the/invention is not to be interpreted asbeing limited in operation in these particular ranges.

Additionally, the receiver described hereinbclow will be of theso-called homodyne type. However, it is again emphasized that thehomodyne type receiver is for illustative purposes only and theinvention is not to be interpreted as being limited to such a receiver.

Accordingly, FIG. 1 illustrates a system constructed in accordance withthe present invention and includes a transmitting unit 10 having anoscillator 12 which generates a signal of fundamental frequency F. Thesesignals are applied to a series chain comprising a directional coupler14, an amplitude modulator l6, and a filter 18. The output signal istransmitted from an antenna 20 which is connected to the filter 18. Aninformation oscillator 22 which generates a modulating signal offrequency f is connected to the amplitude modulater 16. Accordingly,antenna radiates a signal of fundamental frequency F which is amplitudemodulated by the modulating frequency f. It is well known that thisradiation is represented by a carrier of frequency F and two sidebandsof frequency F +fand F f, as shown in FIG. 2A.

A target designated generally by the reference character 24 ispositioned in the path of signals radiated from antenna 20. This targetcomprises an antenna 26 which will receive energy from the radiatedsignals. A harmonic generating element or diode 28 is connected theretowhich receives energy from the antenna 26 and reradiates a harmonicthereof back to a receiving unit designated generally by the referencenumeral 30. A reactive or frequency selective network 32 comprising acapacitor 34 and an inductor 36 is connected across the diode 28.

The output signal produced by the diode 28 will be a distorted waveformof the input signal of frequencies F, F +f, and F f because of theinherent nonlinear characteristics of the diode. Accordingly, the outputsignal will contain a number of harmonics of frequency F, such as, forexample, a signal of frequency 2F. Because of the inherent nonlinearcharacteristics of the diode, as noted in greater detail below, eachsuch harmonic of frequency F will be surrounded by sidebands separatedfrom each other and from such harmonic of frequency F by the modulatingfrequencyf. For example, the harmonic signal of frequency 2F will besurrounded by sideband signals of frequency 2F if, 2F i 2f, 2F i 3fandso on. The amplitudes of these sideband signals decreases withincreasing frequency separation from the harmonic frequency signal andare less than the amplitude of the signals of harmonic frequency thatthey surround. For example. the amplitudes of signals of frequencies 2Fif are less than the amplitude of the signal of frequency 2F, and theamplitudes of signals of frequencies 2F 2f are less than the amplitudesof signals of frequencies 2F if, as shown in FIG. 2B.

In the absence of frequency selective network 32, the ratio of theamplitudes of the signals of frequencies 2F: f, to the amplitudes offrequencies 2F i 2f, is substantially constant. This ratio is altered ifthe impedance of network 32 is different at frequencyfthan it is atfrequency 2f. Such impedance difference is most pronounced when thereactive network 32 is resonant at frequency fand is not resonant atfrequency 2f, as shown in FIG. 2C. Thus, the amplitudes of signals offrequencies 2F-' -f are significantly altered by the reactive network 32whereas the amplitudes of signals of frequencies 2F and 2F 1*: 2farerelatively unaffected by the resonant circuit. Accordingly, the ratio ofamplitudes of signals of frequenceis 2F ifto the amplitudes of signalsof frequencies 2F i 2fis one value for targets which include a frequencyselective network 32 and is another value for all other targets which donot include network 32. To put this another way, the target and thefrequency selective network essentially produce an output signalcomprising a coded arrangement of signal components wherein the networkcontrols the desired coding of the signal components.

At present, it is believed that the reason the coding of the signalcomponents is obtained is due to the fact that the diode, in addition toproducing harmonics of the received signal, also detects the amplitudemodulating signal. This detected signal produces a potential across thenetwork. The network causes an alteration of the amplitude of thispotential in accordance with the impedance of network This potentialacross the network is then applied to the diode and amplitude modulatesthe harmonic signals thereby providing the coded spectrum of signals.

In light of the above, it will be obvious that the diode can be replacedby discrete devices which perform the same function such as a separateand distinct harmonic generator, detector and amplitude modulator.Additionally, the frequency selective network need not be limited toreactive components but may comprise a resitor-capacitor circuit or aresistor'inductor circuit.

A receiving unit is provided which includes a receiving antenna 38 whichis connected to a series chain comprising a filter 40 and a mixer 42. Inview of the fact that the second harmonic of the fundamental F is usedin the illustrative embodiment of the present invention, a frequencydoubler 44 is connected between the directional coupler l4 ane the mixer42. However, if a harmonic other than the second harmonic is utilized,an appropriate frequency multiplier will be used instead of thefrequency doubler 44.

The filter 18 is tuned to the fundamental frequency F and the filter 40is tuned to the second harmonic frequency 2F. Both filters haverelatively wide pass bands so that signals in the vicinity of therespective frequencies F and 2F will not be attenuated. Accordingly, themixer 42 receives a signal of frequency 2F from the frequency doubler 44and signals of frequencies 2F, 2F :t

30 f, and 2F i 2f from the filter 40 and produces, among other signals,a signal having a frequency equal to the difference between thefrequencies of the two signals in the conventional manner. Thisdifference signal contains frequencies f and 2f, the relative amplitudesof which having been previously altered by the network 32 in the target.Signals of frequenciesfand 2f are respectively amplified in tunedamplifiers 46 and 48 and are detected by detectors 50 and 52,respectively, con nected thereto. The ratio of the output signals ofdetec tors 50 and 52 is determined in an amplitude comparator 54, whichratio is equal to the ratio of the amplitudes of signals of frequencies2F ifto the amplitudes of signals of frequencies ZFiZf as radiated bythe target. When this ratio is that of a target with a resonantfrequency selective network 32, such as shown in FIG. 2C, an alarm 56 isactivated by a recognition signal at the output of the comparator 54.When this ratio is that of a target with a different or no network, suchas shown in FIG. 2B, or when no target is present, the alarm is notactivated.

Accordingly, a recognition system has been disclosed which reliablydetects the presence of a particular harmonic generating target althoughother harmonic generating targets may be in the area under surveillance.

An alternate embodiment of a system for recognizing a target whichincludes a network is described hereinbelow. Similar referencecharacters in the figures represent identical elements. The system isshown in FIG. 3 and includes a transmitting unit which is substantiallysimilar to the transmitting unit 10 of FIG. 1 with the exception that asecond local oscillator 58 is provided.

More specifically, oscillators 22 and 58 are connected to amplitudemodulator 16 and generate equal amplitude modulating signals offrequencies f and g. Accordingly, antenna 20 radiates a signal offundamental frequency F amplitude modulated by modulating signals offrequencies f and g, as shown in FIG. 4A. The target 24 reradiatessignals of frequencies 2F, 2F 1 f, 2F g, 2F 3: 2f, 2F t 2g, and so on.Because the higher order mixing components have a lower amplitude,frequencies 2F i 2f, 2F 1 2g, and so on are not considered. Accordingly,the signal reradiated by the target 24 with no frequency selectivenetwork will contain equal amplitude signals of frequencies 2F if and 2Fg, as shown in FIG. 4B. When the network 132 is resonant at frequency g,the amplitude of signal frequencies 2F :t g will be altered, all shownin FIG. 4C and the ratio of the amplitude of signals of frequencies 2F if to the amplitudes of signals of frequencies 2F i g will no longer beunity.

The tuned amplifiers 46 and 48 are tuned to frequencies f and grespectively. These signals are detected in detectors 50 and 52 and theratio of the output signals of detectors 50 and 52 is determined in theamplitude comparator, which ratio is equal to the ratio of theamplitudes of signals of frequencies 2F i f to the amplitudes of signalsof frequencies 2F ig as radiated by the target 24. This ratio is unityunless network 132 is attached to the target. Accordingly, the alarm 56is activated when this ratio is not equal to unity.

Two or more different classes of targets may be separately recognized bythe application of three or more modulating frequencies to the amplitudemodulator 16.

A further modified embodiment of a detection system constructedaccording to the present invention includes a variable frequencyoscillator 60 (FIG. 5) which is connected to the .amplitude modulator 16in the transmitter unit 210. Thus, the output signal radiated from theantenna will be the carrier signal of frequency F modulated by a signalfwhich varies with time between f and f Although the modulating waveformis assumed to be a sinusoidal function for illustrative purposes, it isto be noted that the invention is not limited thereto as the modulatingwaveform may be a triangular wave, etc. However, in accordance with theinvention, at one point the frequency of the modulating waveform must beequal to the resonant frequency f,- of the network 232 of the target 24.Hence, when the instantaneous frequency f of the modulating waveform orsignal f is equal to f,, the amplitude A of the signals of frequencies2F if, that is radiated by the target 24 will be altered, therebycreating a unique response.

To be more specific, as shown in FIG. 6, the carrier signal of frequencyF has an envelope the period of which is l/f,,,. During a cycle of themodulating signal the modulating waveform will sweep through theresonant frequency f, of the tuned circuit 232 twice. As a result, therewill be two amplitude discontinuities per cycle of the modulatingsignal. Accordingly, the period of amplitude modulation, as shown inFIG. 6, is l/2f A tuned amplifier 62 in the receiver 230 amplifies allof the frequency components of modulation frequency f,,, received fromthe target 24. A detector 64 is connected to the output terminals of thetuned amplifier 62 and provides at its output terminals a signal thatvaries in amplitude in proportion to the amplitude A of the signal offrequency 2F if, that is radiated by the target 24. As noted above, thisoutput signal of the detector 64 will have a frequency which is twicethe repetition rate of the modulating signal (i.e., the period of theamplitude signal is l/2f A tuned amplifier 66 is connected to thedetector 64 and amplifies the signals having a frequency of 2f Adetector 68, which is connected to the amplifier 66 detects signals offrequency 2f,,, and actuates the alarm 56 if any such signals aredetected.

FIGS. 7-11 illustrate respective modified embodiments of targets for usewith the systems described and FIGS. 7-9 also show the amplitudecharacteristics of the waveform radiated from the respective target as afunction of the modulation frequency f,,,.

To be more specific, FIG. 7A illustrates a target 70 which includes adiode 72 and an inductor 74 connected thereacross. The amplitude of thesignal 2F if, radiated from the target 70 as a function of themodulating signal f,, is as shown in FIG. 78.

FIG. 8A illustrates a target 76 comprising a diode 78 and a capacitor 80connected thereacross. The amplitude of the signal 2F 1 f,, radiatedfrom the target 76 is shown in FIG. 813 as a function of the signalf,,,.

FIG. 9A illustrates a target 82 comprising a diode 84 and the seriescircuit of an inductor 86 and a capacitor 88 connected thereacross. Theamplitude of the signal 2F if, radiated from the target 82 isillustrated in FIG. 9B as a function of the modulating signal f,,,. Theamplitude reaches a peak at the frequency f, which is the resonantfrequency of the series tuned circuit comprising the capacitor 88 andthe inductor 86.

FIG. 10 illustrates a further embodiment of a target 90 having multiplefrequency selective networks 92A, 92B and 92C connected across a diode94. The network comprises inductors 96A-96C and capacitors 98A-98Crespectively connected in parallel therewith. Connected across each suchnetwork is a respective shorting switch l00A-l00C. When a switch isclosed, the network associated with it is essentially disabled.Moreover, each network may be tuned to a different frequency so that theresponse of any one single target will be unique and will be dependentupon the-fact as to which shorting switches are closed.

That is, with all shorting switches open, the networks will provide aplurality of stop bands. Closing selected ones of the shorting switchesl00A-l00C will alter the pattern of stop bands. The closed shortingswitches may be chosen so that the signal returned by the target is adesired preselected coded arrangement thereby increasing the recognitioncapabilities of the system. It is obvious that more than three networksmay be added to further increase the capability of the system. Moreover,the shorting switches may simply be wires which are cut either by themanufacturer or by the purchaser to enhance security.

FIG. 1 1 illustrates a target 102 which is similar to the target 90 ofFIG. 10 and includes multiple frequency selective circuits which areseries resonant rather than parallel resonant as shown in FIG. 10. Thus,the target 102 includes diode 104 and frequency selective networks106A-106C connected thereacross. The networks respectively comprise theseries circuits of inductors 108A-l08C, capacitors ll0A-110C andswitches 112A-1 12C. When a switch is opened the reactive elementsassociated therewith will be disabled.

FIG. 12 illustrates a modified embodiment of a transmitter l 14constructed according to the present invention. The transmitter 114includes a first RF oscillator 116 which produces a signal F Connectedin series with the oscillator 116 is a filter 118, which passes thefrequency F and an antenna 120 which radiates the signal F, to thetarget.

Additionally, the transmitter includes a second RF oscillator 122 whichproduces a signal F +f,,,. The oscillator 122 is connected to adirectional coupler 124 which connects the signal with the frequencydoubler in the receiver 30 via the lead 126. Also connected to thecoupler 124 is a filter 128 which passes the signal F +f,, to an antenna130.

As is conventional in a two frequency system of the type described, thesignal received by the target will be an amplitude modulated signalwherein the modulations will vary at the frequency f,,,. Thus, thetransmitter 114 may be used in place of a transmitter having anamplitude modulator therein.

While preferred embodiments of the invention have been shown anddescribed herein, numerous omissions, changes and additions may be madein such embodiments without departing from the spirit and scope of thepresent invention.

What is claimed is:

1. An electronic recognition system comprising transmitting means fortransmitting an amplitude modulated signal; target means adapted toreceive said modulated signal for generating a harmonic signal having aplurality of signal components, having respective amplitudes, saidtarget means further comprising amplitude varying means for varying theamplitude of at least one of said plurality of signal components toobtain an output signal having a preselected coded arrangement of theamplitudes of said plurality of signal components; and receiving meansresponsive to said output signal, said receiving means comprisingsensing means for producing a recognition signal when said preselectedcoded arrangement of amplitudes of said plurality of signal componentsis received.

2. An electronic recognition system as in claim 1, in which saidtransmitting means comprises signalproducing means for producing asignal of preselected frequency, and modulator means for amplitudemodulating said signal of preselected frequency.

3. An electronic recognition system as in cliam 1, in which saidtransmitting means includes signalproducing means for generating atleast two signals for different preselected frequencies.

4. An electronic recognition system as in claim 1, in which said targetmeans comprises a harmonic generator for generating at least a harmonicsignal from said modulated signal, detecting means for detecting theamplitude modulating signal, means for applying the detected signal tosaid amplitude varying means, and modulating means for modulating saidharmonic signal with the signal from said amplitude varying means.

5. An electronic recognition system as in claim 1, in which said targetmeans further comprises a diode connected across said amplitude varyingmeans.

6. An electronic recognition system as in claim 1, in which saidamplitude varying means comprises a frequency selective network foraltering at least one of said plurality of signal components.

7. An electronic recognition system as in claim 1, in which saidamplitude varying means comprises a plurality of frequency selectivenetworks for altering different ones of said plurality of signalcomponents.

8. An electronic recognition system as in claim 7, and disabling meansfor selectively disabling predetermined ones of said plurality offrequency selective networks.

9. An electronic recognition system as in claim 1, in which saidamplitude varying means comprises a tuned circuit which is tuned to thefrequency of one of said signal components.

10. An electronic recognition system as in claim 1, in which saidtransmitting means comprises at least one signal generator forgenerating a signal of pre-selected frequency, and said receiving meansincludes multiplying means connected to said transmitting means formultiplying said signal of preselected frequency to produce a componentsignal of said harmonic signal, mixing means for mixing said receivedharmonic signal with said multiplied signal, said sensing meanscomprising comparing means for comparing the amplitudes of said receivedsignals and for producing said recognition signal when the amplitudes ofsaid signal components are in said preselected arrangement.

11. An electronic recognition system as in claim 10, and an alarm deviceresponsive to said recognition signal for producing an indication thatsaid recognition signal has been generated.

12. An electronic recognition system as in claim 2, in which saidmodulating means comprises at least two signal generators for modulatingsaid signal of preselected frequency with modulating signals ofdifferent frequencies.

13. An electronic recognition system as in claim 2, in which saidmodulator means comprises a variable frequency signal generator formodulating said signal of preselected frequency with a signal thefrequency of which varies with time.

14. An electronic recognition system as in claim 13, in which saidsensing means further comprises amplitude responsive means forgenerating a signal proportional to the amplitude of said receivedsignal.

15. An electronic recognition system as in claim 14, and alarm meansconnected to said amplitude responsive means for producing saidrecognition signal when the amplitude of the signal from said amplituderesponsive means reaches a preselected value.

16. A recognition system comprising transmitting means for transmittingan amplitude modulated signal comprising a plurality of signalcomponents, a target comprising harmonic generator means operable toreceive said amplitude modulated signal for generating a harmonic outputsignal, detecting means for producing a detected signal comprising atleast some of the components of the amplitude modulated signal,modulating means for modulating said harmonic output signal with saiddetected signal, and a frequency selective network for altering theamplitude of at least one of the components comprising said detectedsignal to produce a coded arrangement of signal components whichcomprise said harmonic output signal; and receiving means for producinga recognition signal when said coded arrangement of signal components issensed.

17. A recognition system as in claim 16, in which said transmittingmeans comprises oscillator means for generating at least two signals ofdifferent preselected fundamental frequencies, and an antenna forradiating said two signals to produce a composite amplitude modulatedsignal which is modulated by a signal having a fre quency equal to thedifference in frequency between said two signals.

18. A recognition system as in claim 16, in which said harmonicgenerator means, detecting means and mod ulating means comprise a diodeconnected across said frequency selective network.

19. A recognition system as in claim 16, in which said frequencyselective network is a parallel resonant circuit.

20. A recognition system as in claim 16, in which said frequencyselective network is a series resonant circuit.

21. A recognition system as in claim 16, in which said frequencyselective network comprises a resistor connected to a reactive element.

22. A recognition system as in claim 16, in which said frequencyselective network comprises a filter having a plurality of stop bands.

23. A recognition system as in claim 22, and switch means connected tosaid filter for selectively varying the stop bands of said filter.

24. A target for use with a recognition system adapted to produce anamplitude modulated signal at said target, said target comprisingharmonic means for generating a harmonic of said amplitude modulatedsignal, detecting means for detecting the amplitude modulated signal,amplitude varying means for varying the amplitude of preselectedcomponents in said detected signal, and modulating means for modulatingthe harmonic signal with the signal from said amplitude varying means toproduce an otuput signal having a predetermined coded arrangement ofsignal components.

25. A target as in claim 24, in which said' amplitude prise a diode.

1. An electronic recognition system comprising transmitting means fortransmitting an amplitude modulated signal; target means adapted toreceive said modulated signal for generating a harmonic signal having aplurality of signal components, having respective amplitudes, saidtarget means further comprising amplitude varying means for varying theamplitude of at least one of said plurality of signal components toobtain an output signal having a preselected coded arrangement of theamplitudes of said plurality of signal components; and receiving meansresponsive to said output signal, said receiving means comprisingsensing means for producing a recognition signal when said preselectedcoded arrangement of amplitudes of said plurality of signal componentsis received.
 2. An electronic recognition system as in claim 1, in whichsaid transmitting means comprises signal-producing means for producing asignal of preselected frequency, and modulator means for amplitudemodulating said signal of preselected frequency.
 3. An electronicrecognition system as in cliam 1, in which said transmitting meansincludes signal-producing means for generating at least two signals fordifferent preselected frequencies.
 4. An electronic recognition systemas in claim 1, in which said target means comprises a harmonic generatorfor generating at least a harmonic signal from said modulated signal,detecting means for detecting the amplitude modulating signal, means forapplying the detected signal to said amplitude varying means, andmodulating means for modulating said harmonic signal with the signalfrom said amplitude varying means.
 5. An electronic recognition systemas in claim 1, in which said target means further comprises a diodeconnected across said amplitude varying means.
 6. An electronicrecognition system as in claim 1, in which said amplitude varying meanscomprises a frequency selective network for altering at least one ofsaid plurality of signal components.
 7. An electronic recognition systemas in claim 1, in which said amplitude varying means comprises aplurality of frequency selective networks for altering different ones ofsaid plurality of signal components.
 8. An electronic recognition systemas in claim 7, and disabling means for selectively disablingpredetermined ones of said plurality of frequency selective networks. 9.An electronic recognition system as in claim 1, in which said amplitudevarying means comprises a tuned circuit which is tuned to the frequencyof one of said signal components.
 10. An electronic recognition systemas in claim 1, in which said transmitting means comprises at least onesignal generator for generating a signal of pre-selected frequency, andsaid receiving means includes multiplying means connected to saidtransmitting means for multiplying said signal of preselected frequencyto produce a component signal of said harmonic signal, mixing means formixing said received harmonic signal with said multiplied signal, saidsensing means comprising comparing means for comparing the amplitudes ofsaid received signals and for producing said recognition signal when theamplitudes of said signal components are in said preselectedarrangement.
 11. An electronic recognition system as in claim 10, and analarm device responsive to said recognition signal for producing anindication that said recognition signal has been generated.
 12. Anelectronic recognition system as in claim 2, in which said modulatingmeans comprises at least two signal generators for modulating saidsignal of preselected frequency with modulating signals of differentfrequencies.
 13. An electronic recognition system as in claim 2, inwhich said modulator means comprises a variable frequency signalgenerator for modulAting said signal of preselected frequency with asignal the frequency of which varies with time.
 14. An electronicrecognition system as in claim 13, in which said sensing means furthercomprises amplitude responsive means for generating a signalproportional to the amplitude of said received signal.
 15. An electronicrecognition system as in claim 14, and alarm means connected to saidamplitude responsive means for producing said recognition signal whenthe amplitude of the signal from said amplitude responsive means reachesa preselected value.
 16. A recognition system comprising transmittingmeans for transmitting an amplitude modulated signal comprising aplurality of signal components, a target comprising harmonic generatormeans operable to receive said amplitude modulated signal for generatinga harmonic output signal, detecting means for producing a detectedsignal comprising at least some of the components of the amplitudemodulated signal, modulating means for modulating said harmonic outputsignal with said detected signal, and a frequency selective network foraltering the amplitude of at least one of the components comprising saiddetected signal to produce a coded arrangement of signal componentswhich comprise said harmonic output signal; and receiving means forproducing a recognition signal when said coded arrangement of signalcomponents is sensed.
 17. A recognition system as in claim 16, in whichsaid transmitting means comprises oscillator means for generating atleast two signals of different preselected fundamental frequencies, andan antenna for radiating said two signals to produce a compositeamplitude modulated signal which is modulated by a signal having afrequency equal to the difference in frequency between said two signals.18. A recognition system as in claim 16, in which said harmonicgenerator means, detecting means and modulating means comprise a diodeconnected across said frequency selective network.
 19. A recognitionsystem as in claim 16, in which said frequency selective network is aparallel resonant circuit.
 20. A recognition system as in claim 16, inwhich said frequency selective network is a series resonant circuit. 21.A recognition system as in claim 16, in which said frequency selectivenetwork comprises a resistor connected to a reactive element.
 22. Arecognition system as in claim 16, in which said frequency selectivenetwork comprises a filter having a plurality of stop bands.
 23. Arecognition system as in claim 22, and switch means connected to saidfilter for selectively varying the stop bands of said filter.
 24. Atarget for use with a recognition system adapted to produce an amplitudemodulated signal at said target, said target comprising harmonic meansfor generating a harmonic of said amplitude modulated signal, detectingmeans for detecting the amplitude modulated signal, amplitude varyingmeans for varying the amplitude of preselected components in saiddetected signal, and modulating means for modulating the harmonic signalwith the signal from said amplitude varying means to produce an otuputsignal having a predetermined coded arrangement of signal components.25. A target as in claim 24, in which said amplitude varying a frequencycomprises afrequency selective network.
 26. A target as in claim 25, inwhich said amplitude varying means comprises a plurality of frequencyselective networks.
 27. A target as in claim 26, and selectivelyoperable disabling means for disabling desired ones of said plurality offrequency selective networks.
 28. A target as in claim 24, in which saidharmonic means, detecting means and modulating means comprise a diode.